"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Ketone Triplets 43
Table 3.5 Rate constants for the reactions of thiyl radicals with silicon hydrides in
cyclohexane at 60 8C [34]
Silane RS: kSH=M 1 s 1a kSH=M 1 s 1b
Et3SiH 1-AdS: 3:2 104 t-C12H25S: 3:5 104 t-BuMePhSiS: 4:1 104 Ph3SiS: 4:3 104 (t-BuO) 3SiS: 1:3 105 i-Pr3SiS: 1:6 105 Ph3SiH 1-AdS: 7:5 104 Ph3SiS: 8:8 104 t-BuMePhSiH 1-AdS: 3:4 104 3:5 104 n-C12H25S: 1:6 104 t-BuMePhSiS: 5:5 104 Ph3SiS: 5:6 104 (t-BuO) 3SiS: 2:6 105 i-Pr3SiS: 2:5 105 a Method: reduction of 1-bromooctane by silane catalysed by thiols.
b Method: thiol-catalysed racemization of (S)-t-BuMePhSiH.
RS + (S )-t-BuMePhSiH
RS + (R)-t-BuMePhSiH
RSH + (S)-t-BuMePhSi
RSH
Scheme 3.2 Thiol-catalysed radical chain racemization
k inv
+ (R)-t-BuMePhSi
The rate constants of 1-AdS: with Et3SiH and t-BuMePhSiH are relatively
fast taking into consideration the endothermicity of these reactions. The silanethiyl
radicals Ph3SiS: and t-BuMePhSiS: abstract hydrogen with similar rate
constants, whereas the more hindered (t-BuO) 3SiS: and i-Pr3SiS: radicals
show an enhancement in their reactivity, which suggests that entropic effects
may play an important role.
3.5 KETONE TRIPLETS
The excited states of carbonyl compounds are often considered to be similar to
alkoxyl radicals because of the unpaired electron on the oxygen atom. In
particular, the benzophenone n p* triplet mostly reacts in the same manner
as and at a similar rate to t-BuO: radicals.
Absolute rate constants for the reaction of the benzophenone triplet with
Et3SiH, n-C5H11SiH3, PhSiH3, and Cl3SiH have been measured by laser flash